Modern plastic fly lines now account for practically all fly line sales. They generally consist of a polyvinylchloride (PVC) coating over a uniform nylon core. The coating is usually tapered and is the controlling factor for line density and weight. Some fly lines are made to float, others are made to neutral density and yet others are made to sink at various rates. The floating fly lines generally use minute hollow glass balls mixed with the PVC, the balls making up about 30% of the coating. The PVC normally has a natural density of about 1.2 to 1.25, while that of the glass balls is about 0.3. The sinking lines use various fillers, such as solid glass balls or pulverised lead.
This construction has a number of disadvantages. PVC is naturally a rigid material and plasticisers are added to make it supple. However, these slowly leach out and the PVC gradually returns to its rigid state, with the result that the fly lines crack and fall apart. Also, the glass balls make the fly lines stiffer than they would be without them, which results in more work being done to bend them, leaving less energy to project them when casting
The process of manufacture is fairly crude and it is often impossible to predict the weight of a line being made and so keep it to specification. Many lines are made and then classified. Another drawback is that the surface of the line is not smooth; it has many indentations caused by the glass balls, and it is less durable because of the fillers.
The disadvantages are well recognised in existing patents. For example, British Pat. Nos. 1,399,995 and 1,438,918 mention them and try to overcome them. However, all these attempts are along one path and basically work with the original concept and attempt to mitigate the inherent disadvantages.
All fly lines are sold to an AFTM weight standard. This is based upon the weight of the first 30 feet of the line excluding the two foot or so level tip. Typically the AFTM weights are expressed in numbers in the range 1 to 12. Each number has a weight range acceptable to it expressed in grains. For example
______________________________________ AFTM NO. RANGE IN GRAINS GMS. ______________________________________ 5 134-146 8.68-9.46 6 156-168 10.10-10.88 7 177-193 11.46-12.50 ______________________________________
It is extremely difficult to perform adequately to this standard. The customer nevers finds out because he has to cut off 30 feet to discover it! This also makes it difficult for manufacturers, and the common practice is to guess the weight.
To see why the standard is not kept to, consider the difference in grams between AFTM 5 and 6. it is just 0.647989 gms. over 30 feet of line or just 0.0216 gms. per foot on average. With a line having an SG of 1, say, this works out to a tolerance of 0.000709 gms. per centimeter run, correspondinq to a dimensional tolerance of 10 mm. Clearly such tolerances are very difficult to achieve by running the line at constant speed as it is coated through a variable orifice or die. The variables such as room temperature, changing mix of glass balls, and varying viscosities make it impossible by normal methods.
Another advantageous requirement is for the line to taper and in particular to have a `belly` over the middle portion and to reduce smoothly towards the ends. As a further refinement, to be discussed in more detail later, varying the density along the line to a predetermined pattern can also considerably enhance performance.